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"I've been fortunate to work for two mission-driven, world-changing companies, and Square's vision and passion for helping businesses grow is inspiring," he said, in a statement. "I'm thrilled to join the team and help build amazing products that drive economic growth and empower local businesses around the world."

Mr. Rajaram's loss is likely to be keenly felt as Facebook continues to hone its ad model to make it valuable for marketers while still being undisruptive to users and also looks to put controversial new products like video ads into market.

Mike Hudack, who had been a member of Mr. Rajaram's team, will step into his role as head of product management. He had been primarily focused on measurement to gauge the effectiveness of Facebook advertising.

"Gokul has played a valuable part in building our advertising business. We wish him the best of luck in his future endeavors," Facebook said, in a statement.

Square's hiring of Mr. Rajaram, who was previously product management director for Google AdSense and played a role in the acquisition of DoubleClick, hints that the company is interested in building an ad platform, or at the very least in using its transactional data for ad targeting.

As head of software development for Square's point-of-sale system, he would be able to create ways for purchases made in local stores to pipe into a database and become just another -- albeit immensely valuable -- signal to be used in determining who advertisers should aim their ads at and whether those ads led to purchases. For example, a grocery store using Square Register could help Square build consumer segments of people who buy diapers. Square could then bucket those people as new parents, partner with other data providers and publishers to help Pampers target those people with ads online and elsewhere and even relay back to Pampers whether those ads led back to in-store sales.

Essentially Square would be creating a more powerful version of Facebook's Partner Categories program and rival Google and PayPal, whose mobile wallet products could be combined with their respective ad businesses to close the loop on advertising all the way through to sales.

Mr. Rajaram's arrival isn't the company's only notable hire of late. In April it hired Francoise Brougher, formerly Google's VP of SMB global sales and operations, to be its chief operating officer.